This invention relates to an emission computed tomography apparatus for obtaining a scintigram of a subject dosed with radioisotope (RI).
Prior art emission computed tomography apparatuses use an Anger type gamma camera for detecting gamma rays emitted from the interior of a subject (human patient) under examination. The gamma camera head is rotated stepwise or continuous about the subject to collect gamma rays emitted therefrom in 360-degree directions. The gamma ray detection data is processed to reconstruct a tomogram image representing an RI distribution of the subject.
The gamma camera head has a parallel-hole collimator mounted on the front side thereof. Because of the property of the collimator, the resolution of detection is reduced with an increase in the distance between the collimator and subject. With the prior art emission computed tomography apparatus, therefore, the gamma camera head is moved along an elliptical orbit centered at a body axis of the subject, in order to move the camera head as close to the subject as possible so as to improve the detection resolution.
However, when the gamma camera head is moved along an elliptical orbit, a line passing through the center of the effective field of vision of the camera does not always pass through the body axis (center of revolution) of the subject. For this reason, (360-degree direction) projection data cannot be gained for a portion of the subject. Therefore, with the prior art emission computed tomography apparatus, a false image (which is called artifact in this field) is liable to appear in a reconstructed tomographic image of the subject, and spatial resolution is reduced. Therefore, it is difficult to obtain data suitable for diagnosis.